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HYDROGEOLOGICAL INVESTIGATIONS AT THE PROPOSED ALKIMOS WWTP SITE

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REPORT FOR WATER CORPORATION

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OCTOBER 2004

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236.38/04/1

KATHERINE-W

KATHERINE-W

Rockwater Pty Ltd 236.38/04/1

TABLE OF CONTENTS PAGE NO.

1 INTRODUCTION 1

2 HYDROGEOLOGICAL SETTING 1

3 WWTP SITE INVESTIGATIONS 1 3.1 Method 2 3.2 Results 2

3.2.1 Sieve Analyses 2 3.2.2 Pit Soak-Away Tests 2 3.2.3 Ring Infiltrometer Tests 3 3.2.4 Phosphorus Retention Indices 3 3.2.5 Discussion of Results 5

4 NUMERICAL MODELLING OF EFFECTS OF WASTEWATER INFILTRATION 5 4.1 Purpose and Scope 5 4.2 Description of Model 6 4.3 Model Parameters, Boundary Conditions 6 4.4 Model Calibration 7 4.5 Flow and Flow-Path Modelling 7

4.5.1 Modelling Results 8 4.6 Solute Transport Modelling 9

4.6.1 Nitrogen 10 4.6.2 Phosphorus 11

5 EFFECT OF NUTRIENTS ON NEAR-SHORE ENVIRONMENT 13

6 CONCLUSIONS 13

REFERENCES 15 Tables

Table 1 – Pit Soak-Away Test Results 3

Table 2 – Ring Infiltrometer Test Results 4

Table 3 – Phosphorus Retention Indices Results 4

Table 4 – Adopted Aquifer Parameters 7

Table 5 – Results Of Flow and Flow-Path Modelling 8

Table 6 – Nitrogen Loads In Groundwater Discharging To Ocean 10 Figures 1 Location of Proposed Alkimos WWTP 2 Location of Test Sites And Planned infiltration Ponds, Alkimos WWTP


3 Extent Of Model Grid 4 Comparison Between Model-Calculated Summer Groundwater Levels, and Levels

Measured 19 March 1990 5 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 10 ML/d, Without Eglinton Bores Pumping 6 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 10 ML/d, With Eglinton Bores Pumping 7 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 20 ML/d, Without Eglinton Bores Pumping 8 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 20 ML/d, With Eglinton Bores Pumping 9 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 10 ML/d, With Eglinton Bores Pumping, (Revised Borefield Configuration) 10 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 10 ML/d, With Eglinton Bores And Re-Use Bores Pumping (Case 6) 11 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 10 ML/d, With Eglinton Bores And Re-Use Bores Pumping (Case 7) 12 Calculated Groundwater-Level Rise And Flowlines After 13 Years Infiltration At

Up To 20 ML/d, With Eglinton Bores And Re-Use Bores Pumping (Case 8) 13 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 10 ML/d,

Without Eglinton Bores Pumping 14 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 10 ML/d,

With Eglinton Bores Pumping 15 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 20 ML/d,

Without Eglinton Bores Pumping 16 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 20 ML/d,

With Eglinton Bores Pumping 17 Model-Calculated N Concs At Shore West Of WWTP, 10 ML/d Maximum

Infiltration 18 Model-Calculated N Concs At Shore West Of WWTP, 20 ML/d Maximum

Infiltration 19 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 10 ML/d,

With Only 6 mg/L N, & Eglinton Bores Pumping 20 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 10 ML/d,

With Only 6 mg/L N, & Eglinton Bores Pumping, Without Denitrification 21 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 10 ML/d,

With Eglinton And Re-Use Bores Pumping (Case 6) 22 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 10 ML/d,

With Eglinton And Re-Use Bores Pumping (Case 7) 23 Calculated Nitrogen Concentrations After 13 Years Infiltration At Up To 20 ML/d,

With Eglinton And Re-Use Bores Pumping (Case 8)


Appendices I Lithological Descriptions of Pit Samples

II Sieve Analyses, Pit Samples

III Pit Soak-Away Test Results

IV Ring Infiltrometer Test Results

V Phosphorus Retention Indices (PRI) Results

VI Commentary on impacts from nitrogen loading to the shoreline resulting from short-term infiltration to groundwater (Oceanica Consulting report)

Report on Hydrogeological Investigations at the Proposed Alkimos WWTP Site Page 1


1 INTRODUCTION Field investigations were carried out at Alkimos, between Quinns Rocks and Yanchep (Fig.1), planned for a new wastewater treatment plant (WWTP), to assess the infiltration capacity of soils. Seven potential sites for infiltration ponds were investigated, as shown in Figure 2. In addition, a numerical groundwater model was used to predict the impact on groundwater of infiltrating treated wastewater at the site. This report presents the methods and results of the field investigations and the numerical modelling.

2 HYDROGEOLOGICAL SETTING The proposed Alkimos WWTP site is about 1 km from the coast, within recent mobile sand dunes known as the Quindalup Dune System (Safety Bay Sand). The Safety Bay Sand consists of fine to medium grained quartz sand and shell fragments, and overlies calcareous sand and limestone of the Tamala Limestone. These formations comprise the Superficial aquifer. At the Alkimos WWTP site, the Safety Bay Sand is generally unsaturated: the Tamala Limestone crops out in swales in the northern part of the area, particularly near test sites 1 and 2 (Fig. 2). The top of the limestone is usually hard cap-rock, of variable thickness. The Tamala Limestone is karstic in nature, and has high permeability. The water table is between 5 m (Site 4) and 20 m (Site 5) depth (below ground level). Groundwater in the Tamala Limestone is recharged by rainfall infiltration, and flows westwards to discharge to the ocean. Groundwater flow in the formation is largely controlled by the location, and degree of interconnection, of solution channels within the limestone (Davidson, 1995). A study by Barber et al (1990) in an area 10 km south of Alkimos, indicated groundwater flow velocities of between 85 and 335 m/year. Groundwater salinity in the Superficial aquifer at the WWTP site is about 500 mg/L TDS, increasing to 1,000 mg/L near the coast. Background nutrient concentrations in the area are low: nitrate concentrations are about 1 mg/L, and phosphorus concentrations are less than 0.03 mg/L (Davidson 1995, Plates 60 and 61).

3 WWTP SITE INVESTIGATIONS Seven potential areas for the location of infiltration ponds, within swales around the planned WWTP, were selected for investigation. The test sites are shown in Figure 2.



3.1 METHOD At each site, test pits were excavated to approximately 3 m depth. Material excavated from them was geologically logged, and falling-head (“soak-away”) permeability tests were conducted in them. The test results were analysed using a method given in Sommerville (1986). Representative soil samples were taken for sieve analysis to determine grain-size distribution. Access to the site was difficult: unconsolidated, sandy tracks prevented access of water trucks for the permeability tests. Water was carted to site in a small (1,100 L) tank mounted on the tray of a 4WD utility. Ring infiltrometer tests were carried out at Sites 3 to 7, using a steel ring of 560 mm diameter. Sites 1 and 2, where limestone cap-rock crops out, were not tested. The rings were pushed into the ground to at least 10 cm depth, and soil was tamped on the outside of them, to ensure there was no lateral leakage. After saturating the soil first to eliminate entrapped air, the ring was filled with water and the rate of water-level decline was recorded. The tests were repeated at each site. Permeability was calculated using a method given in Cedergren (1977). 3.2 RESULTS The pits intersected bioclastic and/or quartz-dominated sands, and weakly- to well-cemented (caprock) limestone. Geological logs of soil samples from the test pits are included in Appendix I. 3.2.1 Sieve Analyses Sieve analyses of samples taken from the pits show that the strata consist of moderately- to well-sorted, fine to medium grained sand (Sites 1, 3 (1.0 m), 4 (2.0 m), and 7 (2.5 m)), medium-grained sand (Sites 2, 3 (2.0 m), and 4 (0.6 m)); fine to coarse grained sand (Sites 6 (1.0 m), and 7 (1.0 m)); or medium to coarse-grained sand (Site 5 (1.0 m)). The data are presented in Appendix II. 3.2.2 Pit Soak-Away Tests Results of the pit soak-away tests (Table 1, and Appendix III) suggest there is variable permeability in the study area. Moderate permeabilities were recorded at Sites 5 and 6 (6.5 and 3.4 m/day), and there were moderate to high permeabilities (27, 17 and 26 m/day) at Sites 1, 2 and 7. High permeabilities were measured at Sites 3 and 4 (145 and 80 m/day). The measured permeability values represent both horizontal and vertical components.



At sites 1, 2, and 5 there was cemented, hard caprock limestone, which would limit infiltration unless it is excavated in forming infiltration ponds. Based on the measured permeabilities, the locations most suitable for infiltration of treated wastewater are Sites 3 and 4, followed by Sites 1, 7 and 2. The permeabilities measured at Sites 5 and 6 are also high enough for those sites to be suitable for infiltration ponds. Table 1 – Pit Soak-Away Test Results

MGA94 Coordinates* (Zone 50J)

Permeability Test Pit

mN mE Pit Volume (L)

Infiltration Rate

(L/min) (m/d) (m/sec) 1 6501920 373798 ~1000 66.7 27 3.1 x 10-4

2 6501724 374178 ~1000 16.7 17 1.9 x 10-4

3 6501487 373818 ~1000 127.8 145 1.7 x 10-3

4 6501413 374016 ~1000 100 80 9.3 x 10-4

5 6501367 374387 ~1000 11.6 6.5 7.6 x 10-5

6 6501094 374434 ~1000 7.1 3.4 3.9 x 10-5

7 6501043 374152 ~1000 62.5 26 3.1 x 10-4

*Approx: measured by GPS. MGA94 = Geocentric Datum Australia 3.2.3 Ring Infiltrometer Tests The results of the ring infiltrometer tests (Table 2) indicate variable permeabilities for the surface soils, ranging from 26.5 m/day (Site RIT 3B) to 50.5 m/day (Site RIT 5A). Repeated tests at each site give similar values, except at Site 5 where values of 50.5 m/day and 35.9 m/day were measured. Note that actual values of vertical permeability will be substantially lower than the values calculated from the tests, perhaps one fifth to one tenth of those values, as there is a component of horizontal flow from the rings. The data are presented in Appendix IV. 3.2.4 Phosphorus Retention Indices Fourteen sediment samples (two from each test pit) were analysed to determine Phosphorus Retention Indices (PRI). Consolidated samples were crushed to <2 mm prior to analysis. PRI values were variable, ranging between 2.1 mL/g (Site A5, 1.0 m depth) and 130 mL/g (Site A4, 2.8 m depth). Generally, the samples of calcarenite had higher PRI values, ranging between 9.8 and 70 mL/g, whilst PRI values calculated from sand samples ranged between 2.1 and 13 mL/g (except for a sample from pit A4 at 2.8 m, which had a very high PRI value of 130 mL/g).



Results are presented in Table 3 and the original laboratory report is presented in Appendix V. Table 2 – Ring Infiltrometer Test Results

MGA94 Coordinates* (Zone 50J)

Calculated Permeability Test No.

mN mE

Infiltrometer Volume (L)

Infiltration Rate (L/min) (m/d) (m/sec)

RIT 3A 6501487 373818 42.6 3.22 28.8 3.3 x 10-4

RIT 3B 6501487 373818 43.6 3.23 26.5 3.1 x 10-4

RIT 4A 6501413 374016 45.6 4.48 41.0 4.7 x 10-4

RIT 4B 6501413 374016 43.6 4.84 45.5 5.3 x 10-4

RIT 5A 6501367 374387 35.7 5.1 50.5 5.8 x 10-4

RIT 5B 6501367 374387 38.2 3.58 35.9 4.2 x 10-4

RIT 6A 6501094 374434 49.3 4.22 30.3 3.5 x 10-4

RIT 6B 6501094 374434 49.3 4.08 29.6 3.4 x 10-4

RIT 7A 6501043 374152 34.5 3.83 41.9 4.8 x 10-4

RIT 7B 6501043 374152 39.4 3.94 33.1 3.8 x 10-4

*Approx: measured by GPS. MGA94 = Geocentric Datum Australia Table 3 – Phosphorus Retention Indices Results

Pit Sample Depth (m bgl)

Lithology Phosphorus Retention Index

Adsorption Capacity (after Allen & Jeffery, 1990)

A1 1.0 Calcarenite 34 Strong A1 3.0 Sand 12 Moderate A2 1.0 Calcarenite 70 Strong A2 2.5 Calcarenite 13 Moderate A3 1.0 Sand 5.8 Moderate A3 2.0 Sand 4.5 Weak A4 0.6 Sand 5.0 Weak A4 2.8 Sand 130 Very strong A5 1.0 Sand 2.1 Weak A5 3.0 Calcarenite 15 Moderate A6 1.0 Sand 13 moderate A6 2.5 Calcarenite 9.8 moderate A7 0.3 Sand 13 moderate A7 1.0 Sand 10 moderate



3.2.5 Discussion of Results The cap-rock limestone would greatly restrict the infiltration of treated wastewater, and will need to be stripped in forming infiltration ponds. It was removed in digging the test pits at sites 1 and 2, and there was also some at site 5. Permeability values calculated from falling-head test data for the test pits, and for the ring infiltrometers, indicate there is variable permeability, related to factors such as variations in grain-size, sorting, and compaction. The ring infiltrometers tested the surface soil, whereas the falling-head tests in the test pits relate to sub-soil sands. The permeability values from both sets of tests are moderate to high and will not be the main factor limiting infiltration rates around the WWTP site. As at other WWTP sites in the Tamala Limestone, wastewater quality (nutrients and suspended solids), the ability to allow ponds to dry, and the maintenance of pond floors will be the main controlling factors of infiltration capacity. Based on experience at other WWTP’s in the Tamala Limestone, infiltration rates of at least 0.4 m/d, and probably more than 0.5 m/d will be achievable with high quality effluent containing total phosphorus and nitrogen concentrations at 10 mg/L, or less. For example, at the Gordon Road WWTP at Mandurah, infiltration rates are at least 0.48 m/d with treated wastewater of good quality. Prior to the plant upgrade, infiltration rates were much lower. At Geraldton, infiltration rates are believed to be about 0.4 m/d where permeabilities of 5.6 to 7.2 m/d were indicated from ring infiltrometer tests; and 12 to 16 m/d from constant-head permeability tests carried out in auger holes (Rockwater, 1993).

4 NUMERICAL MODELLING OF EFFECTS OF WASTEWATER INFILTRATION

4.1 PURPOSE AND SCOPE Groundwater flow and solute transport modelling was carried out to determine the effects of infiltrating treated wastewater at the site. Calculation of nitrogen loads in groundwater discharging to the ocean, and whether the infiltrated wastewater could flow back to the planned Eglinton production bores, was particularly important. Also, changes to groundwater levels and the fate of phosphorus in the treated wastewater were to be determined.



4.2 DESCRIPTION OF MODEL The Alkimos model was “telescoped off” the Perth Regional Aquifer Modelling System (PRAMS) groundwater model being developed by the Water Corporation and the Department Of Environment. That process produced a sub-set of the main model: for this project the model was reduced to an area centred on the WWTP site and covering 17.5 km north–south by 19.5 km east–west, and the top two layers of the PRAMS model that represent the Superficial formations. The model consists of a rectangular grid of 55 columns and 55 rows, and cell sizes range from 62.5 m by 62.5 m at some of the planned infiltration ponds, to 500 m by 500 m in marginal areas (Fig. 3). It utilises Processing Modflow Pro (Chiang and Kinzelbach, 1991) software that incorporates MODFLOW, finite-difference groundwater modelling software designed by the US Geological Survey (McDonald and Harbaugh, 1988). Model stress periods were selected to alternate between 212 days of summer (October to April), and 153 days of winter (May to September). All of the recharge is assumed to occur during the winter. The flow-path model PMPATH (Chiang and Kinzelbach, 1994) was used to calculate flow paths and travel times from infiltration ponds to the ocean. Solute transport model MT3DMS (Zheng and Wang, 1999) was used to model the transport, dilution and biodegradation of nitrogen, and the adsorption and transport of phosphorus in the groundwater.

4.3 MODEL PARAMETERS, BOUNDARY CONDITIONS Values of vertical and horizontal hydraulic conductivity, specific yield and storage coefficient were initially as for the PRAMS model. It was necessary to vary values of horizontal hydraulic conductivity for the coastal Tamala Limestone during calibration of the model, as described in Section 4.4, below. The values adopted after calibration are given in Table 4. The PRAMS model uses two recharge models coupled to the flow model to provide recharge rates. For the Alkimos model (which doesn’t include the recharge models), Chengchao Xu (pers. comm.) recommended using recharge rates of 179 mm/a for most of the area, and 6 mm/a for pine plantations. These values were adopted. PRAMS includes extraction from a large number of public and private bores, and these were simulated with average summer and winter extraction rates in the Alkimos model. The Alkimos model was also run with and without the 11 planned Eglinton Superficial



formations bores, pumping at an average winter rate of 1,274 m3/d and an average summer rate of 2,410 m3/d, from 2007. Boundaries to the model include constant-head boundaries representing the ocean, and on the eastern side of the model to represent groundwater flow into the modelled area. Both are in Layer 1 only. The other boundaries are assumed to be no-flow boundaries, and there is assumed to be no flow into or out of the Superficial formations from the underlying Mesozoic sediments. Table 4 – Adopted Aquifer Parameters Parameter Layer 1 Layer 2 Coastal Inland Sand Coastal Inland Sand Limestone And Limestone Limestone And Limestone Horizontal Hydraulic Conductivity (m/d) 350 to 900 20 to 35 350 to 900 15 to 25 Vertical Hydraulic Conductivity (m/d) 0.1 to 5 0.5 to 5 0.1 to 0.5 0.5 Specific Yield 0.2 to 0.275 0.2 to 0.275 0.1 to 0.2 0.1 to 0.2

Storage Coefficient N/A N/A 0.0005 to 0.001

0.0005 to0.001

4.4 MODEL CALIBRATION The PRAMS model has been calibrated to regional groundwater levels, but the model-calculated groundwater levels at the WWTP site were too high. It was necessary to increase values of horizontal hydraulic conductivity for the coastal limestone in order to achieve local calibration in the WWTP area. A comparison of model-calculated and observed groundwater levels for the WWTP area, after calibration, is given in Figure 4. There is a close correspondence, considering that three of the groundwater levels were measured on a different day, and the others were probably measured at a different stage of the ocean tide cycle (groundwater levels in the Tamala Limestone are affected by the ocean tides).

4.5 FLOW AND FLOW-PATH MODELLING Eight cases were modelled using the flow and flow-path models:

1. Infiltration with a peak of 9.7 (~10) ML/d after 13 years, and no pumping from Eglinton bores;

2. As above, but with pumping from Eglinton bores; 3. Infiltration with a peak of 19.4 (~20) ML/d after 13 years, and no pumping from

Eglinton bores;



4. As above, but with pumping from Eglinton bores. 5. As for Case 2, except replacing the bore planned to be north-east of the WWTP with

two new bores located north and south of the WWTP. Each of the new bores would be pumped at half the rate of the other Eglinton bores.

6. As for Case 2, but with up to four re-use bores installed down-gradient of the WWTP to extract 8.8 GL/yr from 2008, including 33.6 KL/d in summer and 9.2 KL/d in winter. The bores are to be located to capture as much wastewater flow as possible, and to allow a nominal travel time of two months between infiltration ponds and the bores.

7. As for Case 6, but with re-use bores extracting 15.3 KL/d in summer and 4.2 KL/d in winter in 2008, increasing to 25.9 KL/d in summer and 7.1 KL/d in winter (6.6 GL/yr) in 2020.

8. Similar to Case 7, except wastewater infiltration increasing to 20 ML/d by 2020; and with re-use bores extracting 18.2 KL/d in summer and 5.0 KL/d in winter in 2008, increasing to 33.6 KL/d in summer and 9.2 KL/d in winter (8.8 GL/yr) in 2020.

The models were used to determine changes in groundwater levels resulting from the infiltration, flow paths, and travel times to the coast or to re-use bores. Infiltration ponds used in the modelling were selected according to proximity to the WWTP, and to spread infiltration across the direction of groundwater flow, i.e. in a north-south direction. A maximum infiltration rate of 0.4 m/d was assumed: a minimum number of infiltration ponds were used/assumed in the modelling, and additional ponds were added in the model once the infiltration capacity of the ponds was approached. 4.5.1 Modelling Results The flow modelling results are summarised in Table 5, and are shown in Figures 5 to 12.

Table 5 – Results Of Flow and Flow-Path Modelling

Case Max. Water Travel Time Travel Time (Section 4.5) Level Rise (m) To Coast (Months) To Re-Use Bores (Months)

After 13 Years Infiltration 1 0.4 8 to 10 Not Applicable (N/A) 2 0.2 8 to 10 N/A 3 0.6 4 to 9 N/A 4 0.5 4 to 10 N/A 5 0.2 8 to 10 N/A 6 0.1 >13 Years for N 2 to 3 7 0.1 >13 Years for N 2 to 3 8 0.4 4 to 7 Years for N 2 to 6



Pumping from the Eglinton bores will reduce the degree and extent of mounding that results from wastewater infiltration, but there is indicated to be little effect of the pumping on the minimum travel time from infiltration ponds to the ocean: that time is more dependent on the rate of wastewater infiltration. Flow-path modelling results indicate there is no possibility of groundwater beneath the infiltration ponds being drawn towards the Eglinton bores. Even if the bores were pumped at their planned peak rates of extraction, and if two bores are located close to the WWTP (Case 5, Fig. 9), all groundwater beneath the ponds would continue to flow towards the ocean. Extraction from re-use bores (Cases 6 to 8) would capture much of the groundwater containing treated wastewater, and greatly increase travel time to the coast. The results of the solute-transport modelling (Section 4.6) indicate that capture by the re-use bores, and the additional time available for denitrification, would mean that nitrogen in the treated wastewater would not reach the coast within 13 years in Cases 6 and 7; and would first reach the coast after four to seven years in Case 8, the timing depending on bore layout and numbers. In practice, more than four re-use bores would be needed to extract up to 8.8 GL/yr, to minimise drawdowns and the possibility of up-coning of saline groundwater from beneath the saltwater wedge. The additional bores would also be more efficient at capturing groundwater containing treated wastewater. Also, some of the re-use bores would need to be abandoned and others constructed further to the west, as additional infiltration ponds are commissioned west of the WWTP.

4.6 SOLUTE TRANSPORT MODELLING Eleven cases were run using the MT3DMS solute-transport model: Cases 1 to 8 were as described above, with nitrogen (or phosphorus) source concentrations assumed to be 10 mg/L (Cases 1 to 4) and 6 mg/L nitrogen for Case 5. The three additional cases were as follows:

9. As for Case 2 (up to 10 ML/d infiltration, and pumping from Eglinton bores in the positions originally planned), but with nitrogen source concentration of 6 mg/L;

10. As for Case 9, but with no denitrification occurring in the aquifer; 11. A run to determine the loadings of nitrogen in groundwater discharging to the ocean, with

the background nitrate concentration of 1 mg/L (= 0.2 mg/L nitrogen). Dispersion was assumed to be zero.



4.6.1 Nitrogen The first-order reaction rate for denitrification was assumed to be 0.006 day-1, the value determined in calibrating the solute transport model for the Gordon Road WWTP at Mandurah, also in an area underlain by Tamala Limestone.

Cases 1 to 4 (10 mg/L Source Concentration) The denitrification and dilution by groundwater throughflow and recharge result in decreasing nitrogen concentrations in groundwater towards the coast (Figures 13 to 16). On discharge to the ocean, concentrations are indicated to be up to 1.2 mg/L for the 10 ML/d case, and up to 3 mg/L for the 20 ML/d case. Plots of variation in nitrogen concentration at a point on the coast (Figures 17 and 18) reflect the gradual increase in wastewater infiltration rates. The curves are irregular because of seasonal changes in recharge, bore pumping, and groundwater throughflow; as well as some (minor) numerical instability. In both the 10 ML/d and 20 ML/d cases, extraction from the Eglinton bores reduces nitrogen concentrations – more so in the 10 ML/d case. Model-calculated rates of groundwater discharge along the coast, and nitrogen concentrations, were used to determine the additional total nitrogen loads in groundwater discharging to the ocean after 13 years of infiltration. Background nitrogen concentrations were assumed to be negligible. The results are presented in Table 6. The nitrogen-enriched groundwater would extend over about 1.5 km (10 ML/d case) or 2 km (20 ML/d case) of coastline. The calculated nitrogen loads in groundwater discharging to the ocean will be used by others to assess the potential impact on the coastal ecology. Table 6 – Nitrogen Loads In Groundwater Discharging To Ocean

Case N Loading

(Sections 4.5 After 13 Years

Infiltration and 4.6) (kg/d)

1 10.5 2 8.9 3 38.7 4 33.4 5 5.3 6 0 7 0 8 9.4 (Winter Only) 9 4.8 10 55.7 11 4



Cases 5, 9 and 10 (6 mg/L Source Concentration) The results indicate that with denitrification (Cases 5 and 9), nitrogen concentrations would be up to 0.6 mg/L above background concentrations in groundwater discharging to the ocean (Fig.19). This would add about 5 kg/d of nitrogen in groundwater discharging to the ocean. Changing the position of Eglinton bores near the WWTP has no significant effect on nitrogen loads: the small difference in the Case 5 and 9 results is due to numerical instability of the MT3DMS model. Without any denitrification (Case 10), nitrogen concentrations would be up to 4.0 mg/L above background concentrations in groundwater discharging to the ocean (Fig. 20). The additional loading of nitrogen in groundwater discharging to the ocean would be about 56 kg/d.

Case 11 (Background Nitrogen Concentrations) With background nitrogen concentrations of 0.2 mg/L, there would be about 4 kg/d of nitrogen in groundwater discharging along the length of coast that would be affected by the 10 ML/d wastewater infiltration.

Cases 6 to 8 (With Re-Use Bores) With the re-use bores pumping at the stipulated rates, nitrogen would not reach the ocean at concentrations above background levels within the 13-year period simulated in Cases 6 and 7 (Figs. 21 and 22). In Case 8, some nitrogen would reach the coast from the northern part of the plume in winter after four years, and from the rest of the plume after seven years. All of the nitrogen would be captured during the summer throughout the 13-year period simulated, because of higher rates of extraction from the re-use bores, and lower rates of groundwater throughflow. In winter, there would be about 9.4 kg/d of nitrogen discharging to the ocean, over about 1.3 km of coastline (Fig.23). 4.6.2 Phosphorus The retardation of phosphorus in aquifers can be modelled using adsorption isotherms such as the non-linear Freundlich isotherm, and this method was used in the Alkimos solute-transport model. Phosphorus retention indices (PRI) measured for sand and limestone at the site can be used to calculate the Freundlich adsorption coefficient (A) for input into the solute transport model, using the following formula (Gerritse, pers. comm.):



A = PRI {1000/(100+5*PRI)}1-b1

(1) Where b1 is an experimentally derived exponent. Gerritse (1996) has derived values for A and b1, for various Western Australian soils, using a time-dependent Freundlich adsorption isotherm: ∆Cs = A (∆C)b1 tb2 (2) Where ∆Cs = change in the sorbed concentration of phosphorus (mg/kg) with time, ∆C = the change in the concentration of phosphorus in solution (mg/L) with time t and b2 = an empirical exponent. Values of ∆Cs and ∆C are then used to calculate the retardation of phosphorus in the aquifer material. The MTD3MS solute transport-modelling package includes an option to use an equilibrium Freundlich isotherm to calculate retardation, using the following equation:

Cs = A Cb1 (3) Gerritse (1996) calculated an adsorption coefficient A = 30 L/kg and a Freundlich exponent b1 = 0.4 from laboratory experiments on a calcareous sand. The bulk density of the sand was 1.45 kg/L. These values were used in the model. The modelling method tends to over-estimate phosphorus concentrations in groundwater, because the Freundlich adsorption coefficient (A) and exponent (b1) used in the modelling have been calculated for a time-dependent isotherm, where the total amount of phosphorus adsorbed increases with time. More phosphorus is adsorbed the longer groundwater is in contact with aquifer material. The equilibrium Freundlich isotherm used by MT3DMS assumes that adsorption has gone to completion, limiting the effects of retardation to the continuing adsorption/desorption process. The impact of using an equilibrium Freundlich isotherm can be seen in modelling results for phosphorus transport from the Esperance WWTP (Rockwater, 2002): the calculated phosphorus concentration after 25 years of infiltration, 100 m down-gradient of the WWTP, was 6 mg/L, an order of magnitude greater than the concentration of 0.6 mg/L measured in a monitoring bore. The Alkimos solute-transport model was run to simulate a 100-year period with a worst-case infiltration of 20 ML/d from day one for the entire period, without the Eglinton bores pumping. The results suggest that it would take about 28 years for phosphorus to first reach the coast, and after 100 years, phosphorus concentrations in groundwater at the coast would be 7 to 8 mg/L. As stated above, we expect actual travel times to be much greater, and



concentrations much smaller than indicated by the modelling. The model can be calibrated to observed concentrations after, say, 10 years of operation, so that better predictions can be made.

5 EFFECT OF NUTRIENTS ON NEAR-SHORE ENVIRONMENT The modelling results indicate that without extracting groundwater for re-use, relatively small quantities of nitrogen originating from infiltration ponds will reach the ocean. Much larger quantities discharge with groundwater to the ocean in many parts of the coastal plain where elevated concentrations of nitrate occur naturally in the groundwater. If groundwater is extracted for re-use, most or all of the nitrogen could be captured. A preliminary report by Oceanica on the potential impact on the near-shore environment of groundwater discharge containing nutrients is included as Appendix VI. It states that:

• There are a number of offshore reefs within 2 km of the beach west of the WWTP. • The EPA has general requirements to maintain or improve water quality, and to not

adversely affect seagrass or other benthic habitat. • An appropriate response would be to describe the existing marine environment, any

increase in nutrient concentrations likely to occur, and the effects of these increases. Groundwater flows will enter the ocean through the intertidal zone, and will be dispersed by a prevailing northerly current along the coast. The near-shore water will be well mixed vertically by the swell and the wind.

6 CONCLUSIONS The planned WWTP site is underlain by sand and limestone that are generally of moderate to high permeability, that will enable treated wastewater to be infiltrated to groundwater from ponds in swales, with only minor mounding of the water table. In three swales, hard cap-rock of low permeability outcrops, or occurs at shallow depth. This will need to be excavated in forming infiltration ponds. Based on rates achieved at other WWTP’s in the Tamala Limestone, infiltration rates of at least 0.4 m/d, and probably more than 0.5 m/d, should be sustainable with the planned high quality of the treated wastewater. The infiltration rates will be limited by the wastewater quality, cycling of ponds, and the maintenance of pond bases rather than the intrinsic permeability of soils at the site.



The results of numerical flow and solute transport modelling of the infiltration planned for the first 13 years of operation, and groundwater flows, indicates the following: • Pumping from the planned Eglinton Superficial formations borefield will not induce

flows of groundwater containing treated wastewater to the borefield, but will reduce the degree of mounding beneath infiltration ponds, and the rate and concentrations of nutrients moving towards the coast.

• The travel time from beneath the infiltration ponds to the coast will range from four to ten months, depending on pond location, maximum infiltration rate (10 or 20 ML/d), and whether or not the Eglinton bores are pumping.

• The maximum groundwater-level rise beneath the WWTP will be between 0.2 m and 0.6 m, again depending on the above factors.

• Nitrogen-enriched groundwater will discharge over 1.5 to 2 km of coastline west of the WWTP, with peak total nitrogen concentrations in the groundwater of between 0.6 and 3 mg/L.

• Additional nitrogen loads in groundwater discharging at the coast will be between 5 and 39 kg/d after 13 years of infiltration (depending on the above factors, and nitrogen concentrations of the treated wastewater). At present, about 4 kg/d of naturally occurring nitrogen is discharging along the length of coastline that would be affected with 10 ML/d infiltration.

• Extraction of groundwater down-gradient of the WWTP for re-use, could prevent most or all of the nitrogen entering the groundwater from infiltration ponds from reaching the ocean. The effectiveness of extraction in capturing groundwater elevated in nitrogen will depend on the number of bores and seasonal pumping rates, and the infiltration rate of treated wastewater.

• The transport of phosphorus in groundwater is difficult to predict accurately, because the adsorptive capacity of the sand and limestone is variable and uncertain. Modelling results suggest that at 20 ML/d continuous infiltration and without the Eglinton bores pumping, phosphorus in groundwater would first reach the coast after about 28 years, and after 100 years phosphorus concentrations in groundwater at the coast would be around 7 to 8 mg/L. However, a comparison of modelling results and observed concentrations in a similar hydrogeological environment suggests that the model over-estimates phosphorus concentrations by an order of magnitude. The Tamala Limestone generally has a high adsorptive capacity, and elevated phosphorus concentrations are rarely seen in groundwater from the formation.

• A preliminary report by Oceanica suggests that an investigation should be carried out to characterise the near-shore environment west of the WWTP, and to assess the potential impact of nutrients in groundwater discharging at the coast.



Dated: 11 OCTOBER 2004 Rockwater Pty Ltd C E S New Hydrogeologist P H Wharton Principal Hydrogeologist

REFERENCES Allen, D. G. and Jeffery, R. C., 1990, Methods for Analysis of Phosphorus in Western Australian Soils. Report of Investigation No. 37, Chemistry Centre of Western Australia, March 1990. Barber, C., Davis, G.B., and Buselli, G., 1990, Development of procedures for more efficient monitoring and assessment of groundwater contamination from point sources of pollution: Final Report to AWRAC, EPA, WAWA. The Mindarie Regional Council and The Atlas Group. Cedergren, H.R., 1977, Seepage, drainage and flow nets. John Wiley & Sons (Second Edition). Chiang W.H., and Kinzelbach, W., 1991, Processing Modflow (PM), Pre- and post-processors for the simulation of flow and contaminant transport in groundwater system with MODFLOW, MODPATH and MT3D. Distributed by Scientific Software Group, Washington, DC. Chiang W.H., and Kinzelbach W, (1994), PMPATH. An advective transport model for Processing Modflow and Modflow. Geological Survey of Hamburg, Germany. Davidson, W. A., 1995, Hydrogeology and groundwater resources of the Perth Region, Western Australia: Western Australian Geological Survey, Bulletin 142. Gerritse, R.G., 1996, Transport times of dissolved inorganic phosphate in soils. Journal of Environmental Quality Vol. 25, No.1.



McDonald, M.G., and W.A. Harbaugh, 1988, MODFLOW, A Modular Three-Dimensional Finite-Difference Ground-Water Flow Model. U.S. Geological Survey, Washington, DC. (A:3980), open file report 83–875, Chapter A1. Rockwater, 2002, The results of additional groundwater flow, and solute transport modelling for proposed wastewater treatment plant No.2, Esperance. Unpub. Rept. to Water Corporation. Rockwater, 1993, Geraldton WWTP No2, Capacity for infiltration of treated wastewater, and groundwater effects. Unpub. Rept. to Water Authority of WA. Sommerville, S.H., 1986, Control of groundwater for temporary works. Pub. CIRIA, London. Zheng, C., and Wang, P.P., 1999, MT3DMS: A modular three-dimensional multispecies model for simulation of advection, dispersion and chemical reactions of contaminants in groundwater systems; documentation and user's guide. Contract report SERDP-99-1, U.S. Army Engineer Research and Development Center, Vicksburg, MS.


FIGURES

CLIENT: Water Corporation

PROJECT: Alkimos WWTP

DATE: August 2004

Dwg. No: 236.38/04/1-1

LOCATION OF PROPOSED

ALKIMOS WWTP

Rockwater Pty Ltd

FIGURE 1

!

alkimos.tif/Fig1locmap.srf

368000 370000 372000 374000 376000 378000 380000 382000m E (AMG)

6494

000

6496

000

6498

000

6500

000

6502

000

6504

000

6506

000

6508

000

6510

000

m N

(AM

G)

0 2000 4000

��Alkimos WWTP study area



DATE: August 2004

Dwg. No: 236.38/04/1-2

LOCATION OF TEST SITES

AND PLANNED INFILTRATION PONDS

ALKIMOS WWTP

Rockwater Pty Ltd

FIGURE 2

!

373500 374000 374500 375000

m E (MGA)

6501000

6501500

6502000

m N

(MG

A)

Site 1

Site 2

Site 3

Site 4Site 5

Site 6Site7

0 500 1000pitlocs.xls/alkimos_1.bln/Fig2locmap.srf

PlannedWWTP



DATE: August 2004

Dwg. No: 236.38/04/1-3

EXTENT OF MODEL GRID

Rockwater Pty Ltd

FIGURE 3

!

grid.dxf/.srf

368000 370000 372000 374000 376000 378000 380000 382000 384000

m E (MGA)

6494

000

6496

000

6498

000

6500

000

6502

000

6504

000

6506

000

6508

000

6510

000

6512

000

m N

(MG

A)

0 2000 4000

Alkimos WWTP



DATE: August 2004

Dwg No: 236.38/04/1-4

FIGURE 4

COMPARISON BETWEEN MODEL-CALCULATED

SUMMER GROUNDWATER LEVELS

(CONTOURS, m AHD) AND LEVELS MEASURED

19 MARCH 1990 (BORE VALUES)

rlwls.xls/sssumwl.dat/.grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500m

N (M

GA)

1.13

1.20

0.41

1.12

1.05

0.76

1.00

1.12

1.21

WWTPSite



DATE: August 2004

Dwg No: 236.38/04/1-5

FIGURE 5

CALCULATED GROUNDWATER-LEVEL RISE (m) AND

FLOWLINES AFTER 13 YEARS INFILTRATION AT UP

TO 10 ML/d, WITHOUT EGLINTON BORES PUMPING

s2610mdd.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite

Planned Eglinton Bore

Each Arrow Head On Flowlines Denotes Two Months Flow



DATE: August 2004

Dwg No: 236.38/04/1-6

FIGURE 6



TO 10 ML/d, WITH EGLINTON BORES PUMPING

s2610medd.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite





DATE: August 2004

Dwg No: 236.38/04/1-7

FIGURE 7



TO 20 ML/d, WITHOUT EGLINTON BORES PUMPING

s2620mdd.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite





DATE: August 2004

Dwg No: 236.38/04/1-8

FIGURE 8

CALCULATED GROUNDWATER-LEVEL RISE (m)

AND FLOWLINES AFTER 13 YEARS INFILTRATION

AT UP TO 20 ML/d, WITH EGLINTON BORES PUMPING

s2620medd.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite





DATE: September 2004

Dwg No: 236.38/04/1-9

FIGURE 9

CALCULATED GROUNDWATER-LEVEL RISE (m)

AND FLOWLINES AFTER 13 YEARS INFILTRATION

AT UP TO 10 ML/d, WITH EGLINTON BORES PUMPING

(REVISED BOREFIELD CONFIGURATION)

13y10nbdd.dat/grd/13ynbpth.dxf/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite





DATE: October 2004

Dwg No: 236.38/04/1-10

FIGURE 10



TO 10 ML/d, WITH EGLINTON BORES

AND RE-USE BORES PUMPING (CASE 6)

re-use1dds.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite


Modelled Re-Use Bore

Each Arrow Head On Flowlines Denotes One Month Flow



DATE: October 2004

Dwg No: 236.38/04/1-11

FIGURE 11





ru2dds.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite






DATE: October 2004

Dwg No: 236.38/04/1-12

FIGURE 12





s26ru3dds.dat/grd/.srf

Rockwater Pty Ltd!

372000 372500 373000 373500 374000 374500 375000 375500 376000 376500m E (MGA)

6499000

6499500

6500000

6500500

6501000

6501500

6502000

6502500

6503000

6503500

6504000

6504500

m N

(MG

A)

WWTPSite






DATE: August 2004

Dwg No: 236.38/04/1-13

FIGURE 13

CALCULATED NITROGEN CONCENTRATIONS (mg/L)

AFTER 13 YEARS INFILTRATION AT UP TO 10 ML/d,

WITHOUT EGLINTON BORES PUMPING

s2610mn.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite




DATE: August 2004

Dwg No: 236.38/04/1-14

FIGURE 14



WITH EGLINTON BORES PUMPING

s2610men.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite




DATE: August 2004

Dwg No: 236.38/04/1-15

FIGURE 15



WITHOUT EGLINTON BORES PUMPING

s2620mn.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite




DATE: August 2004

Dwg No: 236.38/04/1-16

FIGURE 16



WITH EGLINTON BORES PUMPING

s2620men.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite


Client: W

ater Corporation

Project: Alkimos W

WTP

Date: August 2004

Dw

g. No: 236.38/04/1-17

MO

DEL-C

ALCU

LATED N

CO

NC

S (mg/L)

AT SHO

RE W

EST OF W

WTP

10 ML/d M

AXIMU

M IN

FILTRATIO

N

FIGU

RE 17

Rockw

ater Pty Ltd

nconcs.xls/.grf

!

0 1000 2000 3000 4000 5000Days From 1 September 2007

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Nitrogen C

oncentration (mg/L)

With Eglinton Bores Pumping From 2010Without Eglinton Bores

Client: W

ater Corporation

Project: Alkimos W

WTP

Date: August 2004

Dw

g. No: 236.38/04/1-18

MO

DEL-C

ALCU

LATED N

CO

NC

S (mg/L)

AT SHO

RE W

EST OF W

WTP

20 ML/d M

AXIMU

M IN

FILTRATIO

N

FIGU

RE 18

Rockw

ater Pty Ltd

20m13yn.xls/.grf

!

0 1000 2000 3000 4000 5000Days From 1 September 2007

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Nitrogen C

oncentration (mg/L)

With Eglinton Bores Pumping From 2010Without Eglinton Bores




Dwg No: 236.38/04/1-19

FIGURE 19



WITH ONLY 6 mg/L N, & EGLINTON BORES PUMPING

13y6n.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite





Dwg No: 236.38/04/1-20

FIGURE 20



WITH ONLY 6 mg/L N, & EGLINTON BORES PUMPING

WITHOUT DENITRIFICATION

13y6n-dn.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite




DATE: October 2004

Dwg No: 236.38/04/1-21

FIGURE 21



WITH EGLINTON AND RE-USE BORES PUMPING

(CASE 6)

RU1s26n.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite





DATE: October 2004

Dwg No: 236.38/04/1-22

FIGURE 22




(CASE 7)

ru2n.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite





DATE: October 2004

Dwg No: 236.38/04/1-23

FIGURE 23




(CASE 8)

s26ru3n.dat/grd/.srf

Rockwater Pty Ltd!

372000 373000 374000 375000 376000m E (MGA)

6499000

6500000

6501000

6502000

6503000

6504000

m N

(MG

A)

WWTPSite




APPENDICES


APPENDIX I

LITHOLOGICAL DESCRIPTIONS OF PIT SAMPLES, ALKIMOS


APPENDIX I Lithological Descriptions of Pit Samples,

Alkimos SITE 1

Depth (m) Lithology Description

0 to 0.5 m Sand with calcarenite rubble

Greyish brown, moderately sorted, medium grained, subangular to subrounded, quartz sand. Some iron staining, minor organic matter. Unconsolidated, with calcarenite rubble (as below).

0.5 to 2.0 m Calcarenite Greyish cream, moderately sorted, fine to medium grained, subangular to subrounded, quartz with calcite cement, minor heavy minerals, hard, some fractures.

2.0 to 3.3 m Sand Cream, moderately sorted, fine to medium grained, subangular to subrounded, quartz and carbonate (skeletal) grains, weakly cemented.

SITE 2

Depth

(m) Lithology Description

0 to 0.1 m Sand Greyish brown, moderately sorted, medium grained, subangular to subrounded, quartz sand. Some iron staining, minor organic matter, unconsolidated.

0.1 to 1.6 m Calcarenite Cream, moderately sorted, fine to medium grained, subrounded, quartz and carbonate (skeletal) grains, calcite cement, hard.

1.6 to 3.0 m Calcarenite Creamy orange, moderately- to well-sorted, medium grained, subangular to subrounded, quartz sand. Iron stained, calcite cement, moderately hard.

SITE 3


0 to 0.6 m Sand

Dark grey, moderately sorted, fine to medium grained, subangular to rounded, quartz and carbonate (skeletal) grains. Some iron staining, minor organic matter, unconsolidated.

0.6 to 2.3 m Sand Cream, moderately- to well-sorted, fine to medium grained, subrounded to rounded, quartz and carbonate (skeletal) grains. Some iron staining, unconsolidated.


SITE 4


0 to 0.3 m Sand Greyish brown, moderately sorted, fine to medium grained, subangular to rounded, quartz and carbonate (skeletal) grains. Minor organic matter, unconsolidated.

0.3 to 1.0 m Sand

Cream, moderately sorted, medium grained, subrounded to rounded, quartz and carbonate (skeletal) grains. Some iron staining, unconsolidated.

1.0 to 1.2 m

Sand Greyish cream, moderately- to well-sorted, medium grained, subrounded to rounded, quartz and minor carbonate (skeletal) grains. Iron stained, unconsolidated.

1.2 to 1.5 m

Sand Cream, moderately sorted, medium grained, subrounded to rounded, quartz and carbonate (skeletal) grains. Some iron staining, unconsolidated.

1.5 to 2.1 m

Sand Grey, moderately sorted, fine to medium grained, subangular to subrounded, quartz and minor carbonate (skeletal) grains. Some iron staining, unconsolidated.

2.1 to 3.2 m

Sand Cream, moderately sorted, fine to medium grained, subangular to subrounded, quartz and carbonate (skeletal) grains, weakly cemented.

SITE 5


0 to 0.3 m Sand Greyish brown, moderately to well sorted, medium to coarse grained, subangular to rounded, quartz sand. Some iron staining, minor organic matter, unconsolidated.

0.3 to 1.2 m Sand Yellow, moderately sorted, medium to coarse grained, subangular to rounded, quartz sand. Some iron staining, unconsolidated.

1.2 to 3.6 m Calcarenite Yellowish cream, moderately sorted, medium grained, subangular to subrounded, quartz with calcite cement. Minor heavy minerals, hard.

SITE 6


0 to 1.7 m Sand Dark grey, moderately sorted, fine to coarse grained, subrounded to well-rounded, quartz and carbonate (skeletal) grains. Minor organic matter, unconsolidated.

1.7 to 2.9 m Calcarenite Pale creamy orange, moderately sorted, medium grained, subangular to subrounded, quartz sand. Iron stained, calcite cement, moderately hard.


SITE 7


0 to 0.6 Sand Black, moderately-poorly sorted, fine to medium grained, silty, quartz and carbonate (skeletal) grains. Carbonaceous, unconsolidated.

0.6 to 1.5 Sand Greyish black, moderately sorted, fine to coarse grained, quartz and carbonate (skeletal) grains. Carbonaceous, unconsolidated.

1.5 to 3.0 Sand Cream, moderately sorted, fine to medium grained, subrounded to rounded, quartz and carbonate (skeletal) grains. Some iron staining, unconsolidated.


APPENDIX II

SIEVE ANALYSIS, PIT SAMPLES, ALKIMOS


Appendix II: Sieve Analysis, Pit Samples, Alkimos CLIENT - Water Corporation: Alkimos WWTP CLIENT No. 236-38

Site 1: 3.0 m Site 2: 2.5 m

µm Mass(g) % cum % µm Mass(g) % cum % >1700 0.2 0.1 0.1 >1700 0 0.0 0.0

1000-1700 0.4 0.3 0.4 1000-1700 0.5 0.3 0.3 500-1000 17 11.0 11.4 500-1000 26 14.6 14.9 250-500 63 40.8 52.1 250-500 116 65.4 80.3 125-250 71 45.9 98.1 125-250 26 14.6 94.9

<125 3 1.9 100.0 <125 9 5.1 100.0 TOTAL 154.6 90% TOTAL 177.5 90%

50% 50% 40% 40%

Site 3: 1.0 m Site 3: 2.0 m

µm Mass(g) % cum % µm Mass(g) % cum % >1700 0 0.0 0.0 >1700 0 0.0 0.0

1000-1700 0 0.0 0.0 1000-1700 0.1 0.0 0.0 500-1000 10 3.5 3.5 500-1000 11 3.5 3.5 250-500 145 50.5 54.0 250-500 193 60.9 64.4 125-250 129 44.9 99.0 125-250 110 34.7 99.1

<125 3 1.0 100.0 <125 3 0.9 100.0 TOTAL 287 90% TOTAL 317.1 90%

50% 50% 40% 40%

Site 4: 0.6 m Site 4: 2.0 m

µm Mass(g) % cum % µm Mass(g) % cum % >1700 0 0.0 0.0 >1700 0 0.0 0.0

1000-1700 0.2 0.1 0.1 1000-1700 0.5 0.2 0.2 500-1000 17 7.0 7.1 500-1000 25 10.7 10.9 250-500 161 66.5 73.6 250-500 125 53.3 64.2 125-250 62 25.6 99.2 125-250 74 31.6 95.7

<125 2 0.8 100.0 <125 10 4.3 100.0 TOTAL 242.2 90% TOTAL 234.5 90%

50% 50% 40% 40%


Appendix II: Sieve Analysis, Pit Samples, Alkimos CLIENT - Water Corporation: Alkimos WWTP CLIENT No. 236-38 Site 5: 1.0 m Site 6: 1.0 m


1000-1700 3 0.9 0.9 1000-1700 0.5 0.2 0.2 500-1000 116 33.2 34.1 500-1000 60 25.9 26.1 250-500 204 58.4 92.6 250-500 126 54.4 80.6 125-250 23 6.6 99.1 125-250 38 16.4 97.0

<125 3 0.9 100.0 <125 7 3.0 100.0 TOTAL 349.1 90% TOTAL 231.5 90%

50% 50% 40% 40%

Site 7: 1.0 m Site 7: 2.5 m


1000-1700 0.8 0.3 0.4 1000-1700 0.5 0.2 0.2 500-1000 56 22.4 22.8 500-1000 45 15.2 15.3 250-500 111 44.4 67.2 250-500 152 51.3 66.6 125-250 66 26.4 93.6 125-250 93 31.4 98.0

<125 16 6.4 100.0 <125 6 2.0 100.0 TOTAL 250 90% TOTAL 296.5 90%

50% 50% 40% 40%

Rockwater Pty Ltd

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Gra

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ize

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)

0102030405060708090100

Cumulative % Retained

Site

1 (3

.0 m

)

Figure A1

236-38/Grapher/Data/Sieve Tests.xls/Sieves Site 1 and 2.grf

Client: Water Corporation Project : Alkimos WWTP Site Date : July 2004 Dwg. No: 236.38/04/1-A1

Grain Size Curves For Pits at Sites 1 and 2

100

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Figure A2

236-38/Grapher/Data/Sieve Tests.xls/Sieves Site 3.grf


Grain Size Curves For Pit at Site 3

100

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0102030405060708090100


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0102030405060708090100


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Figure A3




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APPENDIX III

PIT SOAKAWAY TEST RESULTS


Appendix III: Pit Soakaway Test Results

SITE # 1

(Using Somervilles' Method For Test-pits) Formula: a = 2.72 (Area, m2) k i (t2-t1) = log (h1/h2) - log (αh1+1/αh2+2) p = 6.60 (Pit perimeter, m) h1 = 0.36 (Head at t1, m) where: α = P/2A h2 = 0.30 (Head at t2,m)

(P = mean perimeter, A = area) t1 = 1.0 (Time at h1 in mins) (Assumes hydraulic gradient is unity) t2 = 4 (Time at h2 in mins) t2 - t1 = 3.0 α = 1.213

k 3.0 = 0.0792 - 0.0226 (Interim Calculation) k = 1.89E-02 m/min = 3.14E-04 m/sec Hydraulic Conductivity = 27.17 m/day

(i:\blanks\permsom.xls) Time (min) Time (sec) Head (mm) H (m) 0 0 400 0.4 0.5 30 370 0.37 1 60 360 0.36 2 120 340 0.34 3 180 320 0.32 4 240 300 0.3 5 300 280 0.28 6 360 260 0.26 7 420 240 0.24 8 480 190 0.19 9 540 110 0.11 11 660 50 0.05 15 900 0 0


Appendix III: Pit Soakaway Test Results SITE # 2

(Using Somervilles' Method For Test-pits)

Formula: a = 2.85 (Area, m2) k i (t2-t1) = log (h1/h2) - log (αh1+1/αh2+2) p = 6.80 (Pit perimeter, m) h1 = 0.36 (Head at t1, m) where: α = P/2A h2 = 0.26 (Head at t2,m)



(i:\blanks\permsom.xls) Time (min) Time (sec) Head (mm) H (m) 0 0 370 0.37 0.5 30 360 0.36 1 60 360 0.36 2 120 340 0.34 3 180 330 0.33 4 240 320 0.32 5 300 310 0.31 6 360 310 0.31 7 420 300 0.3 8 480 290 0.29 9 540 270 0.27 10 600 260 0.26 15 900 230 0.23 20 1200 190 0.19 25 1500 160 0.16 30 1800 130 0.13 35 2100 110 0.11 40 2400 80 0.08 45 2700 60 0.06 50 3000 40 0.04 55 3300 20 0.02 60 3600 0 0



SITE # 3




(i:\blanks\permsom.xls) Time (min) Time (sec) Head (mm) H (m) 0 0 195 0.195 0.5 30 177 0.177 1 60 161 0.161 2 120 128 0.128 3 180 100 0.1 4 240 71 0.071 5 300 48 0.048 6 360 25 0.025 7 420 10 0.01 7.83 470 0 0



SITE # 4




(i:\blanks\permsom.xls) Time (min) Time (sec) Head (mm) H (m) 0 0 180 0.18 0.5 30 168 0.168 1 60 156 0.156 2 120 135 0.135 3 180 118 0.118 4 240 100 0.1 5 300 83 0.083 6 360 68 0.068 7 420 50 0.05 8 480 30 0.03 9 540 10 0.01 10 600 0 0


APPIII: Pit Soakaway Test Results SITE # 5


Formula: a = 2.1 (Area, m2) k i (t2-t1) =log (h1/h2) - log (αh1+1/αh2+2) p = 5.8 (Pit perimeter, m) h1 = 0.45 (Head at t1, m) where: α = P/2A h2 = 0.39 (Head at t2,m)



(i:\blanks\permsom.xls) Time (min) Time (sec) Head (mm) H (m) 0 0 460 0.46 0.5 30 455 0.455 1 60 450 0.45 2 120 440 0.44 3 180 435 0.435 4 240 425 0.425 5 300 420 0.42 6 360 410 0.41 7 420 408 0.408 8 480 402 0.402 9 540 395 0.395 10 600 388 0.388 15 900 360 0.36 20 1200 335 0.335 25 1500 310 0.31 30 1800 290 0.29 35 2100 265 0.265 40 2400 240 0.24 45 2700 222 0.222 50 3000 200 0.2 55 3300 180 0.18 60 3600 160 0.16 65 3900 140 0.14 70 4200 120 0.12



Formula: a = 1.96 (Area, m2)

k i (t2-t1) =log (h1/h2) - log (αh1+1/αh2+2) p = 5.6 (Pit perimeter, m) h1 = 0.54 (Head at t1, m)

where: α = P/2A h2 = 0.50 (Head at t2,m) (P = mean perimeter, A = area) t1 = 1.0 (Time at h1 in mins)

(Assumes hydraulic gradient is unity) t2 = 10 (Time at h2 in mins) t2 - t1 = 9.0

α = 1.429


(i:\blanks\permsom.xls) Time (min) Time (sec) Head (mm) H (m)

0 0 550 0.55 0.5 30 545 0.545 1 60 540 0.54 2 120 535 0.535 3 180 530 0.53 4 240 524 0.524 5 300 518 0.518 6 360 514 0.514 7 420 508 0.508 8 480 504 0.504 9 540 500 0.5 10 600 496 0.496 15 900 478 0.478 20 1200 460 0.46 25 1500 440 0.44 30 1800 427 0.427 35 2100 412 0.412 40 2400 400 0.4 45 2700 387 0.387 50 3000 375 0.375 55 3300 360 0.36 60 3600 349 0.349




Formula: a = 3.24 (Area, m2)

k i (t2-t1) = log (h1/h2) - log (αh1+1/αh2+2) p = 7.2 (Pit perimeter, m)

h1 = 0.21 (Head at t1, m) where: α = P/2A h2 = 0.18 (Head at t2,m)

(P = mean perimeter, A = area) t1 = 1.0 (Time at h1 in mins)

(Assumes hydraulic gradient is unity) t2 = 4 (Time at h2 in mins)

t2 - t1 = 3.0 α = 1.111


(i:\blanks\permsom.xls) Time (min)

Time (sec)

Head (mm) H (m)

0 0 310 0.31 0.5 30 260 0.26 1 60 210 0.21 2 120 200 0.2 3 180 190 0.19 4 240 180 0.18 5 300 180 0.18 6 360 160 0.16 7 420 130 0.13 8 480 110 0.11 9 540 100 0.1 10 600 90 0.09 15 900 10 0.01 16 960 0 0


APPENDIX IV

RING INFILTROMETER TEST RESULTS


Appendix IV: Ring Infiltrometer Test Results CLIENT - Water Corporation: Alkimos WWTP CLIENT No. 236-38

Site 3, Test 1 Time (min) Time (sec) Head (mm) H (m) k (m/s) k (m/d)

0 0 173 0.173 Incremental permeabilities

0.5 30 165 0.165 2.52E-04 21.8 1 60 159 0.159 1.97E-04 17.1 2 120 140 0.14 3.39E-04 29.3 3 180 128 0.128 2.39E-04 20.6 4 240 115 0.115 2.85E-04 24.7 5 300 105 0.105 2.42E-04 21.0 6 360 97 0.097 2.11E-04 18.3 7 420 84 0.084 3.84E-04 33.1 8 480 73 0.073 3.74E-04 32.3 9 540 64 0.064 3.51E-04 30.3

10 600 53 0.053 5.03E-04 43.4 Std Deviation 11 660 42 0.042 6.20E-04 53.6 10.89

13.25 795 0 0 Variance Average k (m/d) = 3.33E-04 28.8 118.63



0.5 30 164 0.164 4.07E-04 35.1 1 60 156 0.156 2.67E-04 23.0 2 120 145 0.145 1.95E-04 16.8 3 180 130 0.13 2.91E-04 25.1 4 240 118 0.118 2.58E-04 22.3 5 300 108 0.108 2.36E-04 20.4 6 360 98 0.098 2.59E-04 22.4 7 420 87 0.087 3.17E-04 27.4 8 480 76 0.076 3.60E-04 31.1 Std Deviation

10 600 53 0.053 4.80E-04 41.5 7.46 13.5 810 0 0 Variance

Average k (m/d) = 3.07E-04 26.5 55.68





0.5 30 175 0.175 2.96E-04 25.6 1 60 165 0.165 3.14E-04 27.1 2 120 145 0.145 3.44E-04 29.8 3 180 129 0.129 3.12E-04 26.9 4 240 110 0.11 4.25E-04 36.7 5 300 95 0.095 3.91E-04 33.8 6 360 78 0.078 5.26E-04 45.4 7 420 64 0.064 5.27E-04 45.6 8 480 49 0.049 7.12E-04 61.5 9 540 35 0.035 8.97E-04 77.5 Std Deviation

10 600 17 0.017 1.92E-03 166.3 * 17.06 10.17 610.2 0 0 Variance

Average k (m/d) = 4.74E-04 41.0 290.90



0.5 30 160 0.16 5.38E-04 46.5 1 60 150 0.15 3.44E-04 29.7 2 120 133 0.133 3.21E-04 27.7 3 180 113 0.113 4.34E-04 37.5 4 240 94 0.094 4.91E-04 42.4 5 300 76 0.076 5.67E-04 49.0 6 360 59 0.059 6.75E-04 58.3 7 420 43 0.043 8.43E-04 72.9 Std Deviation 8 480 24 0.024 1.55E-03 134.3 * 14.95 9 540 0 0 Variance Average k (m/d) = 5.27E-04 45.5 223.44

* Anomalous values eliminated in calculating averages, Std deviation and variance





0.5 30 145 0.145 8.48E-04 73.3 1 60 138 0.138 2.64E-04 22.8 2 120 115 0.115 4.86E-04 42.0 3 180 90 0.09 6.53E-04 56.5 4 240 70 0.07 6.70E-04 57.9 5 300 40 0.04 1.49E-03 128.9 * Std Deviation 6 360 10 0.01 3.70E-03 319.3 * 19.03 7 420 0 0 Variance Average k (m/d) = 5.84E-04 50.5 362.11



0.5 30 155 0.155 4.92E-04 42.5 1 60 148 0.148 2.46E-04 21.3 2 120 133 0.133 2.85E-04 24.6 3 180 120 0.12 2.74E-04 23.7 4 240 103 0.103 4.07E-04 35.2 5 300 90 0.09 3.60E-04 31.1 6 360 78 0.078 3.81E-04 33.0 7 420 63 0.063 5.69E-04 49.2 8 480 48 0.048 7.25E-04 62.6 9 540 30 0.03 1.25E-03 108.2 * Std Deviation

10 600 15 0.015 1.85E-03 159.6 * 13.50 10.67 640.2 0 0 Variance

Average k (m/d) = 4.16E-04 35.9 182.22 * Anomalous values eliminated in calculating averages, Std deviation and variance






10 600 35 0.035 9.51E-04 82.1 * 6.46 11.67 700.2 0 0 Variance

Average k (m/d) = 3.51E-04 30.3 41.70




10 600 38 0.038 9.37E-04 80.9 * 9.61 12.08 724.8 0 0 Variance

Average k (m/d) = 3.42E-04 29.6 92.38 * Anomalous values eliminated in calculating averages, Std deviation and variance





0.5 30 130 0.13 3.95E-04 34.1 1 60 122 0.122 3.39E-04 29.3 2 120 115 0.115 1.58E-04 13.6 3 180 95 0.095 5.09E-04 44.0 4 240 75 0.075 6.30E-04 54.4 5 300 60 0.06 5.95E-04 51.4 6 360 45 0.045 7.67E-04 66.3 7 420 30 0.03 1.08E-03 93.4 * Std Deviation 8 480 20 0.02 1.08E-03 93.4 * 17.63 9 540 0 0 Variance Average k (m/d) = 4.85E-04 41.9 310.93



0.5 30 150 0.15 3.44E-04 29.7 1 60 140 0.14 3.68E-04 31.8 2 120 120 0.12 4.11E-04 35.5 3 180 110 0.11 2.32E-04 20.0 4 240 95 0.095 3.91E-04 33.8 5 300 80 0.08 4.58E-04 39.6 6 360 68 0.068 4.33E-04 37.4 7 420 58 0.058 4.24E-04 36.6 8 480 40 0.04 9.90E-04 85.6 * Std Deviation 9 540 25 0.025 1.25E-03 108.2 * 6.13

10 600 0 0 Variance Average k (m/d) = 3.83E-04 33.1 37.58

* Anomalous values eliminated in calculating averages, Std deviation and variance


APPENDIX V

PHOSPHORUS RETENTION INDICES (PRI) RESULTS


Appendix VI: Commentary On Impacts From Nitrogen Loading To The Shoreline Resulting From Short-Term Infiltration To Groundwater (Oceanica Consulting Report)

i:\236-38\raw data\report version of comments mcb 8_10_04 rev3.doc

Alkimos Wastewater Treatment Plant

Commentary on impacts from nitrogen loading to the shoreline resulting from short-term infiltration to groundwater

Prepared for:

Rockwater

Prepared by:

Oceanica Consulting Pty Ltd

October 2004

Report No. 426/1

D R A F T

Revisions history

Submitted to client Report Version Prepared by Reviewed by Copies Date

FINAL 1 M Bailey P Wharton/ K Congdon

1 digital 11/10/04

Disclaimer This report has been prepared on behalf of and for the exclusive use of Rockwater, and is subject to and issued in accordance with the agreed terms and scope between Rockwater and Oceanica Consulting Pty Ltd. Oceanica Consulting Pty Ltd accepts no liability or responsibility whatsoever for it in respect of any use of or reliance upon this report by any third party. Copying this report without the permission of Rockwater or Oceanica Consulting Pty Ltd is not permitted. © Copyright 2004 Oceanica Consulting Pty Ltd

Oceanica: Rockwater: Alkimos Wastewater Treatment Plant i

Contents

1. Introduction ........................................................................................................ 3

2. Background ........................................................................................................ 4

3. EPA Policy .......................................................................................................... 6

4. Existing Environment ........................................................................................ 7

5. Potential impacts of groundwater infiltration ................................................. 9 5.1 Water quality ....................................................................................................................9 5.2 Odour................................................................................................................................9 5.3 Ponding ..........................................................................................................................10 5.4 Suggested strategy for assessment ............................................................................10

6. References........................................................................................................ 11

Oceanica: Rockwater: Alkimos Wastewater Treatment Plant ii

List of Tables Table 2.1 Modelled scenarios ................................................................................. 5 Table 2.2 Discharged nitrogen concentrations and loadings after 13 years of

infiltration. Values for scenarios 1-6 represent loadings above background level (scenario 8)................................................................. 5

List of Figures Figure 1.1 Project location........................................................................................ 3 Figure 5.1 Example of shoreline water quality monitoring data adjacent to

Bunbury WWTP ...................................................................................... 9

Oceanica: Rockwater: Alkimos Wastewater Treatment Plant 3

1. Introduction Rockwater have been engaged by the Water Corporation to examine the impacts of infiltration from the Alkimos WWTP (Figure 1.1) on groundwater levels and quality downstream of infiltration basins. Rockwater have in turn requested that Oceanica provide preliminary comment on the potential magnitude of any impacts of nitrogen rich groundwater on the marine environment and the method by which any future assessment of potential impacts on the marine environment would be undertaken.

Figure 1.1 Project location


2. Background The Water Corporation (2004) submitted a Referral Document to the EPA for the project which contains the following relevant section concerning groundwater infiltration: “It is intended to defer the large capital expenditure required for the construction of the ocean outfall system for approximately 10 years. This will also allow sufficient flows to build up for satisfactory operation of the ocean outlet system at lower flows and velocities. Up to a capacity of 10-15ML/d, it is proposed to recharge the surface aquifer. Following treatment, the wastewater will be discharged to between five and ten infiltration lagoons on Lot 101. These lagoons will generally be sited at lower locations across the site approximately 500m from the shoreline. As far as practical they will be spread in a north-south direction to minimise groundwater mounding. The treated wastewater will be pumped to the basins on rotation, to allow for basin resting and maintenance. ….. This proposal can also be compared to the recently decommissioned system at the Bunbury, where approximately 7ML/d was infiltrated into lagoons located much closer to the shoreline. At Bunbury measurements showed faecal coliform levels along the adjacent shoreline well within the National guidelines for primary contact recreation. At Bunbury the nitrogen and phosphorus (nutrient) levels in the treated wastewater were higher than those found in the natural environment and this resulted in elevated nutrient levels in the nearshore area adjacent to the Bunbury WWTP. At Alkimos, natural groundwater nitrogen concentrations are expected to be higher and the treated wastewater concentrations lower than at Bunbury.” The following information was provided by Phil Wharton of Rockwater: “Evaluating Acceptability of Marine Nitrogen Loading: An evaluation of the acceptability of modelled nitrogen loads to the nearshore environment is required. This needs to consider: • The fact that Lot 101 is adjacent to a fairly enclosed environment, due to reefs

and reef platforms; how much flushing/dilution would be occurring; • The likely DoE position on what would be deemed an ‘acceptable’ load, i.e.

maximum allowable nitrogen load; • That modelled scenarios 5 and 6 are the most likely scenarios; and • That natural groundwater nitrogen concentrations further north are higher

than at the Alkimos site (e.g. Yanchep ~3-6 mg/L TN); could loadings discharged from infiltration be within this natural variation?

This information will be used in evaluating whether infiltration is the best short term disposal option for Alkimos. We are not concerned with an exact, modelled solution, but an assessment on the acceptability of proposed nitrogen loadings. If the answer is not clear, modelling may be undertaken in the future.


Attached (Tables 1 and 2) is a summary of nitrogen loadings (I have removed the no denitrification scenario)”

Table 2.1 Modelled scenarios

Scenario Infiltration in 2020 (ML/day)

Eglinton bores pumping?

Effluent N concentration

(mg/L) Other

1 10 No 10 2 10 Yes 3 20 No 4 20 Yes 10 5 10 Yes 10 NE bore replaced

with 2 bores, each pumping at half the rate of original bore

6 10 Yes 6 8 0 No 0.2 Background GW N

loading

Table 2.2 Discharged nitrogen concentrations and loadings after 13 years of infiltration. Values for scenarios 1-6 represent loadings above background level (scenario 8)

Scenario Max GW level rise

(m)

Travel time to coast

(months)

N loading at coast

(kg/day) N loading

(t/yr) Length of discharge front (km)

N loading (kg/day/km)

1 0.4 8-10 10.5 3.8 1.5 7 2 0.5 8-10 8.9 3.2 1.5 5.9 3 0.6 4-9 38.7 14.1 2.0 19.4 4 0.5 4-10 33.4 12.2 2.0 16.7 5 0.2 8-10 5.3 1.9 1.5 3.5 6 0.2 8-10 4.8 1.8 1.5 3.2 8 0 n/a 4.0 1.5 1.5 2.7


3. EPA Policy The EPA does not have a policy on "Acceptable Nitrogen Loading" as such, rather, there is a general requirement to maintain or improve water quality and then there are criteria for chlorophyll_a (a measure of phytoplankton biomass) which is in turn is usually a measure in response to nitrogen loadings. There is also a general requirement not to adversely affect seagrass or other benthic habitat. The following documents provide guidance on these issues: • Revised Environmental Quality Criteria Reference Document (Cockburn

Sound) (November 2002); and • EPA Guidance Statement 29: Benthic Primary Producer Habitat Protection for

Western Australia's Marine Environment (June 2004).


4. Existing Environment There is limited information at hand to describe the existing marine environment in detail at Alkimos. A brief reconnaissance study was undertaken by DA Lord & Associates (1997). Key findings from this study were: • The study area was characterised by reasonably wide beaches which varied in

width from approximately 100 m (due south of the southern breakwater of the Mindarie Keys Marina) to as little as 20 m in the pocket beach zone immediately north of the breakwater/marina entrance. The average beach width in the study area was in the order of 60 m;

• The beach condition directly opposite Lot 101 was moderately steep. The beach profile was about 1:15 and the materials involved comprised of thick, loosely packed sand;

• There was no exposed reef platform on the beach immediately opposite Lot 101;

• No clear impression was gained of the distribution of seagrass meadows in the nearshore environment. However, from the seagrass mapping that was undertaken by Alan Tingay and Associates (1991) it was assumed that, in common with the rest of Perth’s metropolitan coastal waters, a mosaic of seagrass meadows occurs throughout the study area;

• Throughout the study area, opportunities exist for a wide variety of recreational pursuits, ranging from active sports such as swimming, surfing, diving and angling to more passive forms of recreation such as sunbathing and beachcombing. Due to the shelter offered by fringing reefs, relatively calm and safe bathing conditions occur throughout the study area; and

• There were a number of localities in the study area where emergent reefs occur offshore. It was considered significant from the point of view of the study that about half of these offshore emergent reefs (these being centred upon Pamela Shoal, Eglington Rocks and Alkimos Reef), lay within 2 km of Lot 101. The seven main reefs (from south to north) were: • Burns Rocks—1 km offshore; • Quinns Rocks—1.5 km offshore; • Pamela Shoal—1 km offshore; • Eglington Rocks—750 m offshore; • Alkimos Reef—1.5 km offshore; • Pipidinny Reef—1.3 km offshore; • El Reef—700 m offshore; and • Laurance Reef—450 m offshore.

If this area is typical of the limestone/sand coast elsewhere in the region, then the groundwater flows will enter the ocean through the intertidal zone, possibly with preferred flow pathways through tunnels in karst formations and possibly enhanced flows to the ocean near limestone headlands. Seawater levels will have some affect on flows, with the overall peak flow to the sea likely to be in late winter and spring, primarily due to the effect of winter recharge combined with reducing sea levels as high pressure systems start to dominate the local weather conditions (e.g. Jervoise Bay Groundwater Recovery Scheme, Parsons Brinckerhoff 2003). In relation to the dispersion of groundwater, the waters will be clear, low in nutrients and currents will generally be wind driven, with a prevailing northerly current along


the coast driven by the predominant south-westerly winds. Swell and wind will generally mean that the waters nearshore will be well mixed vertically with longshore currents likely to be in the range of 5 to 15 cm/s.


5. Potential impacts of groundwater infiltration

5.1 Water quality The Water Corporation referral document makes a useful comparison to the Bunbury WWTP, where infiltration of ~7 ML/d occurred to a series of seven ponds between 50 m and 200 m of the beach. Monitoring of the near-shore waters immediately adjacent to the beach found elevated concentrations of bioavailable forms of nitrogen and phosphorus at the sites closest to the Bunbury WWTP and there appeared to be a corresponding response to in phytoplankton growth. Figure 5.1 provides an example of the results. A more detailed investigation of the findings from the three pre-construction surveys for Bunbury may be a useful part of any assessment for Alkimos.

Bunbury October 2000

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

M N A B C D E F G H I J K L

Shoreline Site

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ents

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L)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

Chlo

roph

yll_

a (u

g/L)

FRPNH4NO2+NO3CHLOR-A

Figure 5.1 Example of shoreline water quality monitoring data adjacent to Bunbury WWTP

As stated in the EPA referral document, nitrogen concentrations in treated wastewater are expected to be lower, and the natural groundwater concentrations higher, than at Bunbury. The Rockwater modelling results suggest a relatively small increase in nitrogen concentrations in groundwater discharging at the coast that may be within the range of natural variation in nitrogen concentrations along some sections of the coast. The strength of the long-shore currents, lack of exposed reef platform immediately west of the WWTP site and the high degree of vertical mixing mean that groundwater discharging to the ocean is likely rapidly diluted and dispersed.

5.2 Odour In addition there were effects on local amenity at Bunbury due to odour. Local professional beach fishermen commented on the smell of fish caught in nets immediately offshore the WWTP, while excavation of the beach downstream of Bunbury WWTP would reveal water which had a smell that reflected the higher ammonium concentrations. The additional distance between the Alkimos plant


(600 m to 1,100 m) and the coast is expected to allow all the nitrogen to be oxidised to nitrate and so the are unlikely to be any odour issues. However, this possible concern can be addressed in the approvals documentation.

5.3 Ponding Ponding of water on the beach was one of the key impacts at Bunbury. Apart from odour and community perception of potential health concerns, the elevated groundwater levels were thought to reduce the capacity of the beach to resist erosion at a time when strong storms may still occur (late winter early spring). The greater distance of the Alkimos WTTP from the plant from the ocean means that groundwater levels are not expected to be significantly raised at the coast (Rockwater modelling suggests ~5 cm above background) and so there should be no ponding or instability of the beach at Alkimos.

5.4 Suggested strategy for assessment An appropriate response would be to describe the existing marine environment in terms of nutrient related water quality, residence times, groundwater loadings and benthic habitat and then describe any increase in nutrient concentrations likely to occur and the effects of these increases relative to existing conditions. Given that; • the flows will be similar or larger than those at Bunbury; • the Rockwater model results seem to suggest an increase in nitrogen

concentrations at the coast; • there is reasonable evidence from Bunbury to show the likely nature of any

impacts; and • Then it is recommended that, if the option of infiltration is to be pursued, a

detailed study to address the impacts of nutrients. The potential for ponding and odours on the beach are unlikely to be issues, but should be considered. The tasks would include: • Model likely increases in groundwater levels and changes in nutrient

concentrations; • Review results from Bunbury and Jervoise Bay and any other relevant studies

to develop a likely range of water quality impacts due to groundwater nutrient loads;

• Obtain good background water quality data for the Alkimos shoreline; • Assess the likely residence times of the waters along the shore and the risk of

localised nutrient enrichment; • Assess the any possible impacts on recreational amenity and beach stability;

and • Assess the potential compliance with EPA’s nutrient related water quality

guidelines.


6. References Alan Tingay & Associates Pty Ltd, 1991. Eglington Beach Resort Public

Environmental Review. D.A. Lord & Associates, 1997. Recreational Values Of The Nearshore Marine

Environment Between Burns Beach And Yanchep Beach In The Perth Metropolitan Area. Report to Water Corporation October 1997.

Parsons Brinckerhoff, 2003. Jervoise Bay Recovery Bores, Monitoring Review No.

8, April to September 2003. Report to Dept of Industry and Resources, November 2003.

Water Corporation of Western Australia, 2004. Alkimos Wastewater Treatment

Plant: Wastewater disposal strategy and proposed ocean outlet Referral Document. June 2004

propriet ar y limited - epa.wa.gov.au€¦ · 3.1 method . at each site, test pits were excavated...

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